In order to be considered as suitable replacements for conventional film projectors, digital projection systems must meet demanding requirements for image quality. This is particularly true for multicolor cinematic projection systems. In order to provide a competitive alternative to conventional cinematic-quality projectors, digital projection apparatus must meet high standards of performance, providing high resolution, wide color gamut, high brightness, and frame-sequential contrast ratios exceeding 1,000:1. In addition to these requirements, steps need to be taken to insure the security of the data path and projected images.
The most promising solutions for multicolor digital cinema projection employ, as image forming devices, one of two basic types of spatial light modulators. The first type of spatial light modulator is the digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DMD devices are described in a number of patents, for example U.S. Pat. Nos. 4,441,791; 5,535,047; 5,600,383 (all to Hornbeck); and U.S. Pat. No. 5,719,695 (Heimbuch). Optical designs for projection apparatus employing DMDs are disclosed in U.S. Pat. No. 5,914,818 (Tejada et al.); U.S. Pat. No. 5,930,050 (Dewald); U.S. Pat. No. 6,008,951 (Anderson); and U.S. Pat. No. 6,089,717 (Iwai). Although DMD-based projectors demonstrate some capability to provide the necessary light throughput, contrast ratio, and color gamut; inherent resolution limitations (with current devices providing only 1024×768 pixels) and high component and system costs have restricted DMD acceptability for high-quality digital cinema projection.
The second type of spatial light modulator used for digital projection is the liquid crystal device (LCD). The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. LCDs appear to have advantages as spatial light modulators for high-quality digital cinema projection systems. These advantages include relatively large device size and favorable device yields. Among examples of electronic projection apparatus that utilize LCD spatial light modulators are those disclosed in U.S. Pat. No. 5,808,795 (Shimomura et al.); U.S. Pat. No. 5,798,819 (Hattori et al.); U.S. Pat. No. 5,918,961 (Ueda); and U.S. Pat. No. 6,062,694 (Oikawa et al.).
In an electronic projection apparatus using spatial light modulators, individual colors, conventionally red, green, and blue, are separately modulated in a corresponding red, green, or blue portion of the optical path. The modulated light of each color is then combined in order to form a composite, multicolor RGB color image.
This invention generally relates to an apparatus for displaying a copy protected image while projecting a digital motion picture, where the copy protected image is not significantly degraded as compared to a normally projected image. On the other hand, the copy protected image has a distinguishing attribute that is visible in a recording of the motion picture made using a video capture device such as a video camera.
Whether produced from film or digital sources, images, when projected to a screen for viewing, are subject to illicit duplication. Many techniques have been proposed for a means to prevent off the screen piracy of motion pictures through the use of video recording devices. Illegally copied motion pictures, filmed during projection with video cameras or camcorders and similar devices, are of significant concern to producers of the motion pictures. Even the questionable quality of copies pirated in this fashion does not prevent them from broad distribution. The packaging of these illegal copies can mimic the legitimately distributed media, thus defrauding both the producers and the end users. As video cameras improve in imaging quality and become smaller and more capable, the threat of illegal copying activity becomes more menacing to motion picture providers. While it may not be possible to completely eliminate theft by copying, it can be advantageous to provide display delivery techniques that frustrate anyone who attempts to copy a motion picture using a portable video camera device.
It is known to provide a distinct symbol or watermark to an original still image as a means of image or copy identification, such as in order to authenticate a copy. As examples, U.S. Pat No. 5,875,249 (Mintzer et al.); U.S. Pat. No. 6,031,914 (Tewfik et al.); U.S. Pat. No. 5,912,972 (Barton); and U.S. Pat. No. 5,949,885 (Leighton) disclose methods of applying a perceptually invisible watermark to image data as verification of authorship or ownership or as evidence that an image has not been altered.
The above examples for still-frame images illustrate a key problem: an invisible watermark identifies but does not adversely affect the quality of an illegal copy, while a visible watermark can be distracting and degrades the viewing experience of the intended audience. With video and motion picture images, there can be yet other problems with conventional image watermarking. For example, U.S. Pat. No. 5,960,081 (Vynne et al.) discloses applying a hidden watermark to MPEG data using motion vector data. This method identifies and authenticates the original compressed data stream but would not provide identification for a motion picture that was copied using a camcorder. Other patents, such as U.S. Pat. No. 5,809,139 (Girod et al.); U.S. Pat. No. 6,069,914 (Cox); and U.S. Pat. No. 6,037,984 (Isnardi et al.) disclose adding an imperceptible watermark directly to the discrete cosine transform (DCT) coefficients of a MPEG-compressed video signal. If such watermarked images are subsequently recompressed using a lossy compression method (such as by a camcorder, for example) or are modified by some other image processing operation, the watermark may no longer be detectable.
The watermarking schemes noted above are directed to copy identification, ownership, or authentication. However, even if a watermarking approach is robust, provides copy control management, and succeeds in identifying the source of a motion picture, an invisible watermark may not be a sufficient deterrent for illegal copying. These schemes do not prevent on screen copies to be made, and in addition, require that the watermarking or copy protection be applied to the data stream to the projector.
As an alternative to watermarking, some copy deterrent schemes used in arts other than video or movie display operate by modifying a signal or inserting a different signal to degrade the quality of any illegal copies. The modified or inserted signal does not affect playback of a legally obtained manufactured copy, but adversely impacts the quality of an illegally produced copy. As one example, U.S. Pat. No. 5,883,959 (Kori) discloses deliberate modification of a burst signal to foil copying of a video. Similarly, U.S. Pat. No. 6,041,158 (Sato) and U.S. Pat. No. 5,663,927 (Ryan) disclose modification of expected video signals in order to degrade the quality of an illegal copy.
As a variation of the general method where a signal is inserted that does not impact viewability, but degrades copy quality, U.S. Pat. No. 6,018,374 (Wrobleski) discloses the use of a second projector in video and motion picture presentation. This second projector is used to project an infrared (IR) message onto the display screen, where the infrared message can contain, for example, a date/time stamp, theater identifying text, or other information. The infrared message is not visible to the human eye. However, because the typical video camera has broader spectral sensitivity that includes the IR range, the message can be clearly visible in any video camera copy made from the display screen. The same technique can be used to distort a recorded image with an “overlaid” infrared image. While the method disclosed in U.S. Pat. No. 6,018,374 can be effective for frustrating casual camcorder recording, the method has some drawbacks. A video camera operator could minimize the effect of a projected infrared watermark by applying a commonly available spectral filter designed to block infrared light to the capture lens of his/her camcorder. Video cameras are normally provided with some amount of IR filtering to compensate for silicon sensitivity to IR. Alternately, with a focused watermark image, such as a text message projected using infrared light, retouching techniques could be applied to alter or remove a watermark, especially if the infrared signal can be located within frame coordinates and is consistent, frame to frame.
Motion picture display and video recording standards have well-known frame-to-frame refresh rates. In standard motion picture projection, for example, each film frame is typically displayed for a time duration of 1/24 second. Respective refresh rates for interlaced NTSC and PAL video recording standards are 1/60 second and 1/50 second.
Video camera capabilities such as variable shutter speeds allow close synchronization of a video camera with film projection, making it easier for illegal copies to be filmed within a theater. Attempts to degrade the quality of such a copy include that disclosed in U.S. Pat. No. 5,680,454 (Mead). U.S. Pat. No. 5,680,454, which discloses use of a pseudo-random variation in frame rate, causing successive motion picture frames to be displayed at slightly different rates than nominal. Using this method, for example, frame display periods would randomly change between 1/23 and 1/25 second for a nominal 1/24 second display period. Timing shifts within this range would be imperceptible to the human viewer, but significantly degrade the quality of any copy filmed using a video camera.
Randomization, as used in the method of U.S. Pat. No. 5,680,454, would prevent resynchronization of the video camera to a changed display frequency. While the method of U.S. Pat. No. 5,680,454 may degrade the image quality of a copy made by video camera, this method does have limitations. As noted in the disclosure of U.S. Pat. No. 5,680,454, the range of frame rate variability is constrained, since the overall frame rate must track reasonably closely with accompanying audio. Also, such a method provides no spatial or color disturbance in the illegal copies.
U.S. Pat. No. 5,959,717 (Chaum) also discloses a method and apparatus for copy prevention of a displayed motion picture work. The apparatus of U.S. Pat. No. 5,959,717 includes a film projector along with a separate video projector. The video projector can be used, for example, to display an identifying or cautionary message or an obscuring pattern that is imperceptible to human viewers but can be recorded using a video camera. Alternately, the video camera may even display part of the motion picture content itself. By controlling the timing of the video projector relative to film projector timing, a message or pattern can be made that will be recorded when using a video camera, but will be imperceptible to a viewing audience. The method of U.S. Pat. No. 5,959,717, however, has some drawbacks. Notably, this method requires distribution of a motion picture in multiple parts, which greatly complicates film replication and distribution. Separate projectors are required for the film-based and video-based image components, adding cost and complexity to the system and to its operation. Image quality, particularly for large-screen environments, may not be optimal for video projection and alignment of both projectors to each other and to the display surface must be precisely maintained.
WO 01/33846 A2 (Burstyn) discloses a method and apparatus for anti-piracy that describes an electronic projection apparatus with an interfering source, but it fails to consider the image planes necessary to accomplish the desired interference. The method disclosed by Burstyn does not permit the interference to occur at a plane that is conjugate to the spatial light modulator which is required for projecting an in focus, sharp copy protected image to a screen. As Burstyn is vague concerning the location and design of the interfering means within an electronic projection apparatus, Burstyn does not anticipate either the problems or opportunities related to designing an interfering means into an actual projection apparatus.
Methods such as those described above could be adapted to provide some measure of copy deterrence or watermarking for digital motion pictures. However, none of the methods noted above is wholly satisfactory for the reasons stated. Therefore, there is a need for copy-deterrence techniques that are enabled by internal image digital projector technology. An internal image projection system is ideally suited to the application of interference elements placed at strategic locations in the illumination and imaging optical paths.
The use of an intermediate imaging optical system is known in the design of electronic projection systems. Exemplary prior art systems are described in U.S. Pat. No. 4,836,649 (Ledebuhr et al.); U.S. Pat. No. 5,357,289 (Konno et al.); U.S. Pat. No. 5,907,437 (Sprotberry et al.); U.S. Pat. No. 6,247,816 (Cipolla et al.); and U.S. Pat. No. 6,439,725 (Na). As a particular example, U.S. Pat. No. 5,597,222 (Doany et al.) discloses, for use in a digital projector, an optical relay lens system that is intended to aid in optical tolerance problems and projection lens working requirements. The system of U.S. Pat. No. 5,597,222 provides a single optical relay lens system to create a full color RGB image at unity magnification. This system fails to anticipate many of the advantages a three intermediate image relay optical systems (one per color), each operating at a nominal 2× magnification, provide internal images that are combined prior to a common projection lens. Although the system described in U.S. Pat. No. 5,597,222 lacks many of the advantages of the an internal image projection systems, the projection system of Doany et al. '222 does inherently provides an image plane where the methods disclosed in this application can be applied.
In summary, there is a need for a system to prevent off the screen piracy of motion images which:                Does not degrade the as viewed image        Degrades illicit copies of the viewed image        Is efficient with regard to light throughput        Is easily implemented        Does not require alterations to the motion picture data stream        
A system which can be easily implemented on digital projection designs and which permit physical access to key planes along the optical axis for incorporation of interference elements is desirable. An example of a desirable plane along the optical axis would be a plane conjugate to the imaging device, for example film or spatial light modulator.
It is an object of the present invention to provide a copy-deterrent projection apparatus for projecting a digital motion picture onto a display screen, a disturbance generator capable of obscuring a color, or colors, of illumination temporally or spatially.
Another object of the present invention is to modulate the color channel which has excess illumination to further optimize the projection system.
Yet another object of the present invention to include a method for preventing the removal of the copy protection apparatus.
Thus, it can be seen that there is a need for improvement in illumination and modulation path optics for digital projection that alleviates the inherent angular limitations of lower cost dichroic coatings while providing maximum brightness and color gamut, as well as access to critical points in the system suited to camcorder defeat methods.